A living body of mammals has immune response systems that excludes pathogenic microorganisms (viruses, bacteria, parasites, etc.) or foreign bodies (both are called “antigen” in the following) that have invaded the living body. One of them is called natural immune response system, another acquired immune response system. The former is an exclusion mechanism comprising phagocytosis by phagocytes (polymorphonuclear leukocytes, monocytes, macrophages, etc.), attack by natural killer (NK) cells, and non-specific recognition such as opsonization of antigen by complements. The latter, acquired immune response system, is an exclusion mechanism by lymphocytes (mainly, T cells and B cells) that acquired the specificity to the antigen (namely, activated lymphocytes). B cells that acquired antigen specificity attacks the antigen existing outside of the cells through production of antibodies specific to the antigen. T cells that acquired antigen specificity (namely, activated T cells) are classified into helper T cells and cytotoxic T cells (cytotoxic lymphocyte, CTL). The helper T cells regulate a differentiation of B cells and a production of antibodies, and destroy the antigen cooperating with phagocytes. The latter, CTLs attack virus-infected cells and so on by themselves (Experimental Medicine: SUPPLEMENT, “Bio Science Term Library, Immunity”, Yodosha, pp. 14–17 (1995)).
This acquisition of antigen specificity by T cells (namely, activation of T cells) is initiated through recognition by T cells the antigen presented by antigen-presenting cells (APC) such as macrophage, B cells, or dendritic cells. Antigen-presenting cells process the antigens so incorporated and present these processed antigens through binding them to major histocompatibility complex (MHC). T cells receives primary signal for activation of the cells (or acquisition of specificity) by recognizing the processed antigens presented by antigen-presenting cells through a complex between T cell receptor (TcR) and CD3 antigen existing on the surface of the cell membrane (TcR/CD3 complex).
However, the TcR/CD3 complex-mediated primary signal alone cannot activate T cells sufficiently and leads to unresponsiveness or clonal anergy, so that the cells can not react with any stimulation received thereafter. The autocrine of interleukin 2 (IL-2) is necessary for T cells to be activated, to be differentiated into antigen specific T cell clones, and to be proliferated. In clonal anergy, T cells are inactivated due to no production of IL-2 and no cell division. Namely, the activation of T cells accompanied by production of cytokines such as IL-2 requires the secondary signal following the first signal through TcR/CD3 complex. This secondary signal is called costimulatory signal.
T cells receive this secondary signal and transmit it into the cells by interacting (cell adhesion) with molecules other than MHC on antigen-presenting cells through other molecules other than TcR/CD3 complex on the T cell surface. This secondary signal avoids cell anergy (clonal anergy) and activates the cells.
Although some part of the mechanism of the secondary signal transmission between antigen-presenting cells and lymphocytes such as T cells have not yet been elucidated in detail, studies so far have revealed that an important factor for the secondary signal transmission is the interaction of CD28 (also named Tp44, T44, or 9.3 antigen), which is a cell surface molecule expressed mainly on T cells and thymus cells, with CD80 (also named B7-1, B7, BB1, or B7/BB1), which is a cell surface molecule expressed on antigen-presenting cells (macrophages, monocytes, dendritic cells, and so on etc.) and with CD86 (also named B7-2 or B70), which is also a cell surface molecule on antigen-presenting cells (namely, cell adhesion through the binding between these molecules). Moreover, it has been experimentally elucidated that the interaction of Cytolytic T lymphocyte associated antigen 4 (CTLA-4), whose expression is thought to be enhanced depending on the secondary signal, with the CD80 (B7-1) and CD86 (B7-2) (namely, cell adhesion through the binding between these molecules) also plays an important role in the regulation of T cell activation by the secondary signal. In other words, the regulation of T cell activation by the transmission of the secondary signal involves, at least the interaction between CD28 and CD80/CD86, the enhancement of CTLA-4 expression, which is thought to depend on the interaction, and the interaction between CTLA-4 and CD80/CD86.
CD28 is known to be a costimulator molecule transmitting the secondary signal (costimulatory signal) required for the activation of T cells and for the avoidance of anergy. The secondary signal transmitted by binding this molecule to costimulator molecules, CD80 (B7-1) and CD86 (B7-2), on antigen-presenting cells (namely, cell adhesion through the binding between these molecules), stabilizes mRNA of Th1-type cytokines and consequently promotes production by T cells of a large amount of production of Th1-type cytokines such as Il-2, IFNγ, and TNFα. The expression of CTLA-4 is induced by the primary signal transmitted through TcR/CD3, and the expression is also enhanced by the secondary signal transmitted by the binding between CD28 and CD80. It is being revealed that CTLA-4 receives these signals to work to inhibit T cell function, which is contrary to the activation of T cells by the secondary signal transmitted by CD28.
Human CD28 and CTLA-4 are I-type glycoproteins whose molecular weights are 44 kD and 41 to 43 kD, respectively. Both have an immunoglobulin-like domain, belong to the immunoglobulin superfamily, and have both function as a cell adhesion molecule and function as a signal transmission molecule.
Human CD28 forms a homodimer with a disulfide bond while CTLA-4 exists as a monomer. Both CD28 and CTLA-4 genes are located at “2q33” on human chromosome and “1C” on mouse chromosome, and are composed of four (4) exons. Human CD28 and CTLA-4 are composed of 220 and 223 amino acids, respectively, including the leader sequences, and amino acid homology between them is 20 to 30%.
The ligands for CD28 and CTLA-4 are CD80 (B7-1) and CD86 (B7-2) in human and mice. CTLA-4 has about 20 times as higher affinity to both ligands as CD28. It has been elucidated that the amino acid sequence structures “MYPPPY (Met-Tyr-Pro-Pro-Pro-Tyr)” (SEQ ID NO:18) conserved through animal species is important for the binding of CD28 and CTLA-4 to CD80(B7-1). It has also been reported that, when CD28 is stimulated, PI3 kinase (phosphoinositide 3 kinase, PI3K) associates with the phosphorylated tyrosine residue in a partial sequence “YMNM (Tyr-Met-Asn-Met)” (SEQ ID NO:19) and that CD28 plays an important role in intracellular signal transmission through this “YxxM” structure. Furthermore, it has been reported that CTLA-4 also has a sequence represented by “YxxM,” namely “YVKM (Tyr-Val-Lys-Met)” (SEQ ID NO:20) in its cytoplasmic region and that, after being stimulated, SYP associates with this sequence.
CD28 is expressed specifically in thymocytes and peripheral blood T cells, and CTLA-4 is expressed specifically in activated T cells (Cell Engineering: SUPPLEMENT, “Handbook of Adhesion Molecule”, Shujunsha, pp. 93–102 (1994); ibid. pp. 120–136; Experimental Medicine: SUPPLEMENT, “BIO SCIENCE Term Library, Immunity”, Yodosha, pp. 94–98 (1995); Experimental Medicine: SUPPLEMENT, “BIO SCIENCE Term Library, Intracellular Signal Transduction”, Yodosha, pp. 58–59 (1997); Nihon Rinsho, Vol.55, No.6, pp. 215–220 (1997)).
In the regulation of T cell function (the activation and the inhibition of function of T cells), the importance of interactions among multiple molecules such as costimulator molecules (CD28, CD80 (B7-1), CD86 (B7-2), etc.) and CTLA-4, which cooperates with them, (in other words, cell adhesion through the binding between these molecules) has thus been recognized, and this has been drawn attention to the relationship between these molecules and diseases, and the treatment of diseases by regulating the function of these molecules have been noted.
As described above, although a living body activates its acquired immune response system against antigens that are foreign bodies to the living body (self), it also has immunological tolerance so as to show no immune response against its own component (autoantigen). If immunological tolerance breaks down by some reason, immune response to the autoantigen occurs, autoantigen-reactive T cells are induced by the same mechanism as mentioned above to fall into abnormal state of immunity, and various autoimmune diseases are caused.
In other words, since non-stimulated antigen presenting cells (APC) in normal tissues do not express costimulatory molecules when the immune system of a living body is normal, T cells fall are in the unresponsiveness state to maintain immunological tolerance even if autoantigen-reactive T cells, which reacts with autoantigen, exist. It has been suggested that in abnormal state of immunity, more autoantigen-reactive T cells are activated due to abnormal excess and continuous expression of costimulatory molecules to thereby cause autoimmune diseases.
From such viewpoints recently, many attempts to treat for various autoimmune diseases by modulating the transmission of costimulatory signals, for example, the above-mentioned signal transmission between CD28/CTLA-4 and CD80/CD86, are proposed.
It has been observed CD80, a costimulatory molecule as the ligand of CD28 and CTLA-4, is abnormally expressed in the antigen presenting cells at the nidus of autoimmune disease such as rheumatoid arthritis, multiple sclerosis, autoimmune thyroiditis, allergic contact-type dermatitis, and chronic inflammatory dermatosis such as squamous lichen planus, and psoriasis. From such observation, many attempts to treat various autoimmune diseases by modulating the function of CD80 have been made.
It has been proposed to block the function of CD80, by methods using an antibody against CD80, solubilized protein of CD28 that is a ligand of CD80, and solubilized protein of CTLA-4 that is also a ligand of CD80. Particularly, based on the fact that the binding affinity of CTLA-4 to CD80 is 20 or more times higher than that of CD28, therapeutic attempts using “solubilized CTLA-4,” specifically, the fusion protein (CTLA-4-IgFc) comprising the extracellular domain of “CTLA-4” and the Fc region of human immunoglobulin IgG1, were performed in animal model and clinical tests (Nihon Rinsho, Vol. 55, No. 6, pp. 215–220 (1997)).
As shown in 1 to 5 below, therapeutic effects of CTLA-4-IgFc in model animals of autoimmune diseases has been reported.
1. In a (NZB/NZW)F1 mouse, that is a model for human systemic lupus erythematosus (SLE), the production of autoantibodies and the onset of lupus nephritis were suppressed by administration of CTLA-4-IgFc before the onset, and the pathologic conditions were improved by administration of the drug even after the onset (Science, Vol. 125, p. 1225–1227 (1994)).
2. In experimental allergic encephalomyelitis (EAE), that is a model for multiple sclerosis (MS), the onset was prevented by short-term administration of CTLA-4-IgFc immediately after immunization (J. Clin. Invest., Vol.95, pp. 2783–2789 (1995)).
3. In an NOD (non-obese diabetes) mouse, which is a model for insulin dependent diabetes mellitus (IDDM), the onset rate was remarkably decreased by administering CTLA-4-IgFc to the 2- or 3-week-old female mouse for two weeks (J. Exp. Med. 181:1145–1155, 1995).
4. In rat nephritis by renal glomerulus basement membrane immunity, Goodpasture's nephritis model, the improvement of the symptom has been improved by the administration of CTLA-4-IgFc (Eur. J. Immunol. 24:1249–1254, 1994).
5. In type II collagen-induced arthritis (CIA) using a DBA/1 mouse, that is a model for human rheumatoid arthritis, the onset of arthritis was suppressed by the administering the test drug at the time of immunization and the production of autoantibodies (IgG1 and IgG2) against collagen was inhibited (Eur. J. Immunol. 26:2320–2328, 1996).
The results of the experiments as mentioned above are have not yet clarified in detail the mechanism of the T cell activation by interaction between costimulatory molecules and the related molecules (in other words, cell adhesion through the binding between these molecules). Other unknown molecules may be involved in this mechanism.